Page 99 - Separation process principles 2

64 Chapter 2 Thermodynamics of Separation Operations

component in the mixture. Which components will have a tendency 2.20 The disproportionation of toluene to benzene and xylenes is
to be present to a greater extent in the equilibrium vapor? carried out in a catalytic reactor at 500 psia and 950°F. The reactor
effluent is cooled in a series of heatexchangers for heat recovery
2.16 Acetone, a valuable solvent, can be recovered from air by ab-
sorption in water or by adsorption on activated carbon. If absorption until a temperature of 235°F is reached at a pressure of 490 psia.
is used, the conditions for the streams entering and leaving are as The effluent is then further cooled and partially condensed by the
listed below. If the absorber operates adiabatically, estimate the tem- transfer of heat to cooling water in a final exchanger. The resulting
perature of the exiting liquid phase using a simulation program. two-phase equilibrium mixture at 100°F and 485 psia is then sepa-
rated in a flash drum. For the reactor effluent composition given
Feed Gas Liquid below, use a computer-aided, steady-state simulation program with
Gas Absorbent Out Out the S-R-K and P-R equations of state to compute the component
Flow rate, lbmollh: flow rates in lbmoVh in both the resulting vapor and liquid streams,
Air 687 0 687 0 the component K-values for the equilibrium mixture, and the rate of
Acetone 15 0 0.1 14.9 heat transfer to the cooling water. Compare the results.
Water 0 1,733 22 1,711
Temperature, OF 78 90 80 - Reactor Effluent,
Pressure, psia 15 15 14 15 Component lbmolh
Phase Vapor Liquid Vapor Liquid Hz 1,900
Some concern has been expressed about the possible explosion CH4 215
hazard associated with the feed gas. The lower and upper flamma- C2H6 17
bility limits for acetone in air are 2.5 and 13 mol%, respectively. Is Benzene 577
the mixture within the explosive range? If so, what can be done to Toluene 1,349
remedy the situation? p-Xylene 508

Section 2.5
Section 2.6
2.17 Subquality natural gas contains an intolerable amount of
2.21 For an ambient separation process where the feed and prod-
nitrogen impurity. Separation processes that can be used to remove
ucts are all nonideal liquid solutions at the infinite surroundings
nitrogen include cryogenic distillation, membrane separation, and
temperature, To, (4) of Table 2.1 for the minimum work of separa-
pressure-swing adsorption. For the latter process, a set of typical
feed and product conditions is given below. Assume a 90% removal tion reduces to
of N2 and a 97% methane natural-gas product. Using the R-K equa-
tion of state with the constants listed below, compute the flow rate in
thousands of actual cubic feet per hour for each of the three streams.
Nz CH4
For the liquid-phase separation at ambient conditions (298 K, 101.3
Feed flow rate, lbmollh: 176 704 kPa) of a 35 mol% mixture of acetone (1) in water (2) into 99 mol%
Tc, K 126.2 190.4 acetone and 98 mol% water products, calculate the minimum work
PC, bar 33.9 46.0 in Mikmol of feed. Liquid-phase activity coefficients at ambient
Stream conditions are conditions are correlated reasonably well by the van Laar equations
with A12 = 2.0 and A21 = 1.7. What would the minimum rate of
Feed
work be if acetone and water formed an ideal liquid solutio~,?
(Subquality Product Waste
Natural Gas) (Natural Gas) Gas 2.22 The sharp separation of benzene and cyclohexane by distil-
lation at ambient pressure is impossible because of the formation of
Temperature, OF 70 100 70
an azeotrope at 77.6OC. K.C. Chao [Ph.D. thesis, University of
Pressure, psia 800 790 280
Wisconsin (1956)l obtained the following vapor-liquid equilib-
2.18 Use the R-K equation of state to estimate the partial fugac- rium data for the benzene (B)/cyclohexane (CH) system at 1 atm:
ity coefficients of propane and benzene in the vapor mixture of
Example 2.5.
2.19 Use a computer-aided, steady-state simulation program to es-
timate the K-values, using the P-R and S-R-K equations of state, of
an equimolar mixture of the two butane isomers and the four butene
isomers at 220°F and 276.5 psia. Compare these values with the fol-
lowing experimental results [J. Chem. Eng. Data, 7, 331 (1962)l:

Component K-value
Isobutane 1.067
Isobutene 1.024
n-Butane 0.922
I-Butene 1.024
trans-2-Butene 0.952
cis-2-Butene 0.876